These two articles in leading progressive British newspapers:
John Kampfner's Why I'm backing the Lib Dems | Comment is free | The Guardian, & Chris Huhne's Independent newspaper readers' Q&A set out excellent reasons to vote Liberal Democrat. Only in my view short on emphasis on green issues.
Why I'm backing the Lib Dems
It started with Iraq. But in 2010 Nick Clegg's party has become the natural home for left-liberal Cookites like me
Also see Colin Firth on why he's stopped voting Labour and now supports the Lib Dems. A thoughtful, non-party political view. Summarised at: http://stephentall.org/2010/03/16/colin-firth-on-why-he%e2%80%99s-stopped-voting-labour-and-now-supports-the-lib-dems/